This invention relates generally to conversion of a polycrystalline ceramic body to a single crystal body and specifically to conversion of polycrystalline alumina to single crystal sapphire by doping the alumina with molybdenum or tungsten.
Molybdenum is known as an undesirable trace impurity in melt grown single crystal alumina (sapphire). Molybdenum particles can become entrapped in the sapphire boule and act as defects which deteriorate the optical properties of the sapphire. Therefore, one of ordinary skill in the art would avoid introducing molybdenum into melt grown sapphire.
Sapphire may also be produced by solid state conversion of polycrystalline alumina. A sapphire or single crystal alumina material is defined as a portion of material having one crystal greater than or equal to one centimeter in size in any direction (i.e., a circle having a diameter of xe2x89xa71 cm or an elongated crystal having a length of xe2x89xa71 cm). Solid state conversion of polycrystalline alumina (xe2x80x9cPCAxe2x80x9d) to single crystal alumina by doping the alumina body with certain conversion (i.e., recrystallization) promoting dopants and then heating the doped alumina body to temperatures between 1100 and 2050xc2x0 C. is disclosed in U.S. Pat. No. 5,588,992 (xe2x80x9cthe ""992 patentxe2x80x9d), which is a divisional of U.S. Pat. No. 5,487,353, both incorporated herein by reference in their entirety. The ""992patentxe2x80x99 discloses that the conversion promoting dopants comprise elements such as chromium, gallium, titanium, cerium and vanadium, which assume the +3 valence state in alumina.
While the +3 valence state dopants are acceptable for promoting the conversion of polycrystalline alumina to sapphire, it still requires about 9 hours to completely convert a 5 mm diameter Cr-doped PCA tube to sapphire. As discussed in the U.S. Pat. No. 5,588,992, the conversion rate from PCA to sapphire increases as the concentration of the dopant, such as chromium, is increased. However, if chromium is present in a very high concentration in the PCA, then the PCA is converted to ruby rather than sapphire. Since ruby has a pink color and luminesces in the red spectral range, a ruby lamp arc tube may not be as desirable as a clear lamp arc tube for certain lamp applications.
The present inventors have determined that it would be desirable to improve the reproducibility and speed of the conversion process, as well as to reduce the cost of the process and the amount of dopants required to promote or enhance the conversion. The present invention is directed towards improving the conversion of a polycrystalline ceramic material to a single crystal ceramic material and to a single crystal ceramic material made by the improved conversion process.
In accordance with one aspect of the present invention, there is provided a process for converting a polycrystalline ceramic material to a single crystal material, comprising doping at least a first portion of the polycrystalline ceramic material with a conversion-enhancing dopant having a +6 valence state in the unfired ceramic material, and then heating the polycrystalline ceramic material to convert at least a second portion of the polycrystalline ceramic material to a single crystal ceramic material.
In accordance with another aspect of the present invention, there is provided a process for converting polycrystalline alumina to sapphire, comprising immersing at least a first portion of a polycrystalline bisque-fired alumina body containing at least 150 parts per million (ppm) of magnesium oxide in a liquid solution containing molybdenum in a concentration equal to or greater than 5 ppm and allowing the molybdenum to penetrate into said alumina body, sintering the alumina body, decreasing the concentration of magnesium oxide in the sintered alumina body to a concentration of 60 ppm or less and heating the sintered alumina body to a temperature above one-half of a melting temperature of alumina but below the melting temperature of alumina to convert at least the first portion of the sintered alumina body to single crystal sapphire.
In accordance with another aspect of the present invention, there is provided a ceramic material body comprising at least a first single crystal sapphire portion containing a first concentration of molybdenum or tungsten sufficient to enhance the conversion of polycrystalline alumina to the single crystal sapphire.